Traditionally, the lost-wax casting technique consists in first creating a model made of wax, or any other material that can easily be eliminated at a later stage, of the part to be produced. This model includes an internal part forming a ceramic core, which represents the cavities that one wants to see appear inside the blade. The wax model is then dipped several times in slurries consisting of a suspension of ceramic particles to make a shell mould, by carrying out so-called stuccoing and drying procedures.
The shell mould is then dewaxed, which is a procedure in which the wax, or the material of which the original model is made, is eliminated from the shell. Once the wax has been eliminated, a ceramic mould is obtained whose cavity reproduces all of the blade's shapes and which still contains the ceramic core intended to generate the internal cavities of the blade. The mould is then subjected to a high-temperature heat treatment or “firing”, which provides it with the required mechanical properties.
The shell mould is then ready to manufacture the metal part by casting. After checking the internal and external integrity of the shell mould, the following step consists in pouring a molten metal, which fills the gaps between the internal wall of the shell mould and the core, and then solidifying it. In the field of lost-wax casting, there are currently several solidifying techniques, thus several pouring techniques according to the nature of the alloy and to the expected properties of the part resulting from the casting. This may be directional solidification of columnar structure (DS), directional solidification of single crystals (SX) or equiaxed solidification (EX).
After casting the alloy, the shell is broken using a shakeout procedure. In another step, the ceramic core, which has remained enclosed in the blade obtained, is eliminated chemically. The metal blade obtained is then subjected to finishing procedures used to obtain the finished part.
Examples of how to produce turbine blades using the lost-wax casting technique are provided in the applicant's patent applications FR2875425 and FR2874186.
To form the wax model of the blade, a tooling outfit, or wax injection mould, is used in which the core is placed and then the liquid wax is injected through a channel provided for this purpose.
The search for improved engine performance implies among others more efficient cooling of the turbine blades located downstream of the combustion chamber. In order to meet this requirement, it is necessary to form more elaborate internal cavities inside the blades to circulate the cooling fluid. A distinctive feature of these blades is that they have several metal walls and thus require the production of increasingly complex ceramic cores.
Due to the complexity of the cooling cavities to be formed with their separating walls and their layout, the core is made of several parts that are assembled and bonded. The basic cores are generally connected one to another at the base and at the top. The goal is indeed to control the thickness of the walls and partitions formed when casting. The assembly must enable the core to support the stresses to which it is subjected during the wax injection, dewaxing and casting steps.
The various parts of the core must therefore be placed in a very precise manner one with respect to another inside the wax injection mould and it must be guaranteed that the relative positions of the various parts of the mould are retained. The retention of the various parts of the core as proposed in the current technique consists in achieving a firm connection between these core parts or elements and the ceramic shell. While such a retention can in theory be used to guarantee precise relative positioning of the various core elements, it has been observed that pouring the molten metal leads to a significant thermal expansion of the core elements, which in turn leads to the deformation of some of these elements due to the static connection one with respect to another of the elements making up the core, which contributes to increasing the scrap rate of the blades. In critical cases, one of the core elements may even break, which obviously leads to the scrapping of the blade obtained but also to the manufacturing of a new core, which is both costly and time-consuming.